a. Field of the Invention
This invention relates to a method and apparatus for cleaning vehicle exhaust system particulate filters and other components associated with emissions control from diesel engines. The process may be suitable for cleaning any type of filter provided that it is enclosed by a solid external casing and has rigid filter membrane or cell structure.
b. Related Art
Many modern diesel powered vehicle exhaust systems are provided with filters for removing particulate matter from the vehicle's exhaust stream. These are typically wall flow filters which are cylindrical ceramic or other type of monoliths having elongate channels running along their length. The channels are alternately plugged at a first or second end such that gas entering a channel at one end cannot exit the same channel at its other end. To exit, the gas must pass from one channel to the next through the porous walls of the filter. In doing so, soot and ash within the gas are deposited within and on the walls of the filter.
Modern diesel exhaust systems are fitted with particulate filters that can be categorised as Passive or Active.
With Passive filters, heat and gases from the engine are used to cause the particulate carbon to burn off, thereby regenerating the filter. The amount of soot and ash deposited, and the ability of the filter to regenerate depends on both the condition of the engine and the duty cycle.
At elevated operating temperatures, such filters regenerate themselves and effectively self-clean, because the higher temperatures burn the deposited soot out of the filter. However, with lower exhaust gas temperatures, caused for example by stop/start city driving, or after prolonged use, the filter can become blocked with heavy deposits of particulate carbon. The exothermic reaction that results from rapid burn-off of heavy carbon deposits can be so severe that damage is caused to the filter's integrity. This may have the consequence of just reducing the effectiveness of the filter, but more commonly the filter has to be replaced. Even when ideal conditions exist the filter will slowly become clogged with ash and other non-combustible materials resulting in increased backpressure within the exhaust system that has a detrimental effect on performance and fuel efficiency.
Active filters are suited to engines that typically have short or low performance duty cycles. These filters are used in conjunction with external actions that are implemented to aid combustion of carbon deposited on the filter as they build up. Once again ash and other products of combustion will accumulate on the filter, slowly reducing performance and fuel efficiency.
One way to reduce these problems is through a regular maintenance regime which includes periodic cleaning. Preferably cleaning of the filter is performed once a year, but depending on usage, the filter may be cleaned two or three times a year.
Existing cleaning methods are time consuming and can be difficult to implement. One standard method used to clean a filter involves introducing a jet of compressed air into one or both ends of the filter to blow out the soot and ash. However, the compressed air often does not fully clean the filter, resulting in reduced service life and increasing the likelihood of premature failure due to irreversible blockage or thermal damage. It has been proposed in EP 1 336 729, to clean a vehicle exhaust filter by first filling the filter housing with a flushing agent heated to 85° C. The filter is soaked with the hot flushing agent for 3-8 minutes, and agitated with a vibrator, followed by expulsion of the flushing agent with compressed air. In EP 1 252 919 it is proposed to carry out a plurality of successive cleaning cycles, during each of which a cleaning liquid at a feed pressure is brought into contact with the whole of the outlet face of the filter medium by means of a gas at a first pressure above atmospheric, after which a cleaning pressure is exerted by means of a gas at a higher pressure than the first pressure.
The invention seeks to provide an improved method and apparatus for cleaning vehicle exhaust system filters.